LED lamp with prismatic cover lens

ABSTRACT

The LED lamp includes a plurality of light emitting diodes and a cover lens arranged so that light emitted by the plurality of light emitting diodes is output through the cover lens. The cover lens has a plurality of prisms including a plurality of first prisms formed on the inner surface thereof, and a direction and shape of the incident surfaces of the first prisms are set such that the first prisms output light in first luminous intensity distribution patterns that are substantially equal to each other.

TECHNICAL FIELD

The present invention relates to an LED lamp and, particularly, to anLED lamp for a traffic signal.

BACKGROUND ART

As light emitting diodes capable of emitting light of R, G and B primarycolors and light emitting diodes capable of emitting white light withhigh luminance have been developed, LED lamps including a plurality oflight emitting diodes arranged in an array have been put in use forvarious applications. The LED lamp has a far higher service life thanthat of an incandescent lamp, and also shows a high efficiency and ahigh resistance against vibration. Due to these advantages, the LED lamphas been used in advertising sign boards, traffic sign boards displayingroute guide or traffic information, and light source for traffic signalsand large screens.

With regard to the application of the LED lamp to traffic signals, inparticular, while the incandescent lamp used as the light source of theconventional traffic signal requires large reflector mirrors and colorfilters, the LED lamp has such advantages as the capability to emitlight of a single color that eliminates the need for a color filter andthe capability to emit light with some degree of directivity thateliminates the need to install a large reflector mirror.

Moreover, a traffic signal constituted from LEDs that does not needreflector mirrors and color filters also has an advantage of being freefrom spurious lighting which is caused by extraneous light that hasentered the traffic signal and been reflected off the reflector mirrorplaced behind an incandescent lamp, coming out of the traffic signalthrough a color filter.

The LED lamp used in such a traffic signal or a sign board is usuallyinstalled at an overhead location so as to be recognized by many peoplefrom a distance. As such, the LED lamp is required to emit light withsymmetrical intensity distribution in the horizontal plane butasymmetrical intensity distribution in the vertical plane so that lightintensity is higher in the front field and the lower field.

As it has been made possible to increase the luminous intensity of lightemitting diodes recently, it is possible to decrease the number of lightemitting diodes required in an LED lamp. However, a new problem hasarisen in that it is difficult to achieve planar light emission ofuniform intensity with an LED lamp consisting of a small number of lightemitting diodes that have high luminous intensity.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an LED lamp that iscapable of achieving a desired luminous intensity distributioncharacteristic.

The second object of the present invention is to provide an LED lampthat is capable of achieving a desired luminous intensity distributioncharacteristic and producing uniform planar light emission.

In order to achieve the objects described above, the LED lamp of thepresent invention comprises a plurality of light emitting diodes and acover lens so that light emitted by the plurality of light emittingdiodes is output through the cover lens.

The cover lens has a plurality of prisms including a plurality of firstprisms formed on the inner surface thereof. The direction and shape ofthe incident surfaces of the first prisms are set such that the firstprisms output light in first luminous intensity distribution patternsthat are substantially equal to each other.

The LED lamp of the present invention that has the constitutiondescribed above can emit light from the plurality of light emittingdiodes in a luminous intensity distribution pattern that corresponds tothe direction and shape of the incident surfaces of the first prisms.

The LED lamp of the present invention also allows the direction in whichlight that is output through the first prisms emerges in the verticalplane to be set by the direction of the incident surface, and thedirection in which light that is output through the first prismsdiffuses in the horizontal plane to be set by the configuration of theincident plane.

The LED lamp of the present invention preferably also includes aplurality of second prisms, and the direction and shape of the incidentsurfaces of the second prisms are set so that the second prisms havesecond luminous intensity distribution patterns that are substantiallyequal to each other.

Since the desired luminous intensity distribution pattern of the LEDlamp can be achieved by means of the first and second prisms in theconstitution described above, a better luminous intensity distributionpattern that is optimized can be achieved than in the case where theluminous intensity distribution pattern is formed with the first prismsonly.

This constitution allows the direction in which light that is outputthrough the second prisms emerges in the vertical plane to be set by thedirection of the incident surface, and the direction in which light thatis output through the second prisms diffuses in the horizontal plane tobe set by the configuration of the incident surface.

In the LED lamp of the present invention, it is preferable that thefirst prisms and the second prisms form a pair, and the pair of prismsare arranged on the inner surface of the cover lens according to acertain rule, thereby achieving a uniform planar light emission on theexternal surface of the cover lens.

In the LED lamp of the present invention, the second prisms can beformed so as to direct the light below the first prisms, which makes itpossible to provide an LED lamp that is required to emit light withhigher intensity in the lower portion.

Further the plurality of light emitting diodes are preferably disposedin an arrangement pattern determined according to the luminous intensitydistribution pattern required of the LED lamp. Since the desiredluminous intensity distribution characteristic of the LED lamp can beachieved based on the arrangement pattern of the plurality of lightemitting diodes, the first luminous intensity distribution pattern andthe second luminous intensity distribution pattern in the constitutiondescribed above having a better luminous intensity distribution patternthat is optimized to provide the luminous intensity distributioncharacteristic described above.

Also in the LED lamp of the present invention, the arrangement patternis preferably set such that the number of light emitting diodes locatedbelow the horizontal plane that includes the central axis of the coverlens is set to 40% or less of the total number of the light emittingdiodes. Thus, the LED lamp that is required to emit light with higherintensity in the lower portion can be made easily.

Also in the LED lamp of the present invention, the angle between theline that connects the light emitting diode located at the outermostposition of the arrangement pattern and the periphery of the cover lenson the inner surface thereof and the central axis of the cover lens ispreferably in a range from 30° to 60°. With this constitution, uniformplanar light emission can be achieved on the external surface of thecover lens.

Moreover, in the LED lamp of the present invention, it is preferablethat the cover lens has an external surface that is a spherical surfacehaving a radius of curvature of 500 mm or less.

With this constitution, when the first prisms or both the first prismsand the second prisms are used, light emitted by the light source can beoutput more efficiently. Also, it is possible to mitigate the reflectionof extraneous light on the cover lens surface, thereby preventing thecontrast from decreasing when lighted.

As described above, the present invention can provide an LED lamp thatis capable of achieving a desired luminous intensity distributioncharacteristic and producing uniform planar light emission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of the LED lamp according to anembodiment of the present invention.

FIG. 2 is an enlarged perspective view of prisms formed on the innersurface of a cover lens according to an embodiment of the presentinvention.

FIG. 3 is a sectional view taken along lines A-A′ in FIG. 2.

FIG. 4 schematically shows the luminous intensity distribution patternof light output through first prisms of this embodiment.

FIG. 5 schematically shows the luminous intensity distribution patternof light output through second prisms of this embodiment.

FIG. 6 is a plan view of an LED assembly 1 of this embodiment.

FIG. 7 schematically shows the relative positions of the LED assembly 1and a cover lens 3 of this embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings.

As shown in FIG. 1, the LED lamp of this embodiment comprises a case 2having a truncated conical shape with a circular base surface and anopening that is parallel to the base surface and has a diameter largerthan that of the base surface, an LED assembly 1 that has a plurality oflight emitting diodes 11 disposed on a substrate 12 which is attached tothe base surface of the case 2, and a cover lens 3 attached to theopening of the case 2. Thus, light is emitted in a predeterminedluminous intensity distribution pattern for an application to a trafficsignal.

For the purpose of forming the predetermined luminous intensitydistribution pattern with the LED lamp of this embodiment, the coverlens 3 has a plurality of prisms 30 comprising a set of first prisms 31and set of second prisms 32 formed on the inner surface thereof, asshown in FIGS. 2 and 3.

The direction and shape of the incident surfaces of the first prisms 31are set so that the first prisms output light in intensity distributionpatterns that are substantially equal to each other. The direction andshape of the incident surfaces of the second prisms 32 are set so thatthe second prisms 32 output light in intensity distribution patternsthat are substantially equal to each other.

In the LED lamp of this embodiment, the configuration of the secondprisms 32 is determined so as to direct light in a direction lower thanthat of the first prisms 31. Specifically, the cover lens 3 of the LEDlamp has one luminous intensity distribution pattern (first luminousintensity distribution pattern) formed by the assembly of the pluralityof first prisms 31, and another luminous intensity distribution pattern(second luminous intensity distribution pattern) formed by the assemblyof the plurality of second prisms 32 so as to form a luminous intensitydistribution pattern as a whole (total luminous intensity distributionpattern) through the superposition of the first and second luminousintensity distribution patterns.

In the LED assembly 1 of the LED lamp, the light emitting diodes 11 arearranged in a pattern that is determined according to the luminousintensity distribution pattern required of the LED lamp.

Thus the luminous intensity distribution pattern of the LED lamp in thisembodiment is constituted by (1) the arrangement pattern of theplurality of light emitting diodes 11, (2) the assembly of the pluralityof first prisms 31, and (3) the assembly of the plurality of secondprisms 32.

Now construction of components and the principle of forming the luminousintensity distribution pattern in the LED lamp according to theembodiment of the present invention will be described in more detailbelow.

(Prisms 30)

The prisms 30 comprise a set of the first prisms 31 and a set of thesecond prisms 32 disposed in a regular pattern on the inner surface ofthe cover lens 3, as shown in FIG. 2. In this embodiment, the firstprisms 31 and the second prisms 32 are arranged alternately in thevertical direction and in the horizontal direction in a regular pattern.

In this specification, taking the situation where the LED lamp isusually used into consideration, the direction in the lateral planeparallel to the central axis of the cover lens 3 is referred to as thehorizontal direction, and the direction perpendicular to the horizontalplane is referred to as the vertical direction. According to thisdefinition, the direction of line A-A′ in FIG. 2 is the verticaldirection. Thus, the prisms 30 comprise the first prisms 31 and thesecond prisms 32 that are disposed adjacent to each other in thevertical direction.

(First Prisms 31)

In this embodiment, each of the first prisms 31 has an incident surfaceconsisting of a part of a cylindrical surface. The inclination of theincident plane of each of the first prisms 31 is set such that lightthat is emitted by a light source located on the central axis of thecover lens 3 (light source located at a position where the LED assembly1 is disposed) and is incident on the first prisms 31 at the center C31thereof emerges, after passing through the first prisms 31, in adirection parallel to the central axis of the cover lens 3(schematically shown in FIG. 3 with reference numeral 101).

The radius and length of the circumference of the cylindrical surfacethat constitutes the incident surface of each of the first prisms 31 areset such that light that is incident on the first prisms 31 and haspassed therethrough spreads to a predetermined extent in the horizontalplane, while the longitudinal length of the cylindrical surface thatconstitutes the incident surface in the direction perpendicular to thecircumferential surface of the cylinder that constitutes the incidentsurface is set such that light that is incident on the first prisms 31and has passed therethrough spreads to a predetermined extent in thevertical plane (plane perpendicular to the horizontal plane). Thecylindrical surfaces that constitute the incident surfaces of the firstprisms 31 are also formed to be substantially equal to each other.

The extent to which light incident on the incident surfaces of the firstprisms 31 in the horizontal direction and in the vertical direction isset in accordance with the luminous intensity distribution patternrequired of the LED lamp in order to form the luminous intensitydistribution pattern required of the LED lamp. The luminous intensitydistribution patterns of the first prisms 31 can be made equal to eachother by forming the cylindrical surfaces that constitute the incidentsurfaces of the first prisms 31 substantially equal to each other andsetting the inclination of the incident surface of each of the firstprisms 31 such that light that is emitted by a light source located onthe central axis of the cover lens 3 is incident on the first prisms 31at the center C31 thereof and emerges therefrom in a direction parallelto the central axis of the cover lens 3. In other words, the directionand shape of the incident surfaces of the first prisms are set so thatthe luminous intensity distribution patterns of light emerging from thefirst prisms 31 are equal to each other in this embodiment.

FIG. 4 shows light emerging through first prisms 31 a, 31 b of twoprisms 30 a, 30 b that are arbitrarily selected from among the pluralityof prisms 30. As shown in FIG. 4, in this embodiment, the luminousintensity distribution patterns formed by the first prism 31 a and theluminous intensity distribution patterns formed by the first prism 31 bare formed so as to have the same extent (cover the same area) in thehorizontal direction and in the vertical direction around the centralaxis of the cover lens 3 located at the center thereof. Meanwhile, theluminous intensity distribution patterns formed by the first prism 31 aand the luminous intensity distribution patterns formed by the firstprism 31 b are substantially equal to each other in terms of thedirection and the pattern.

Although FIG. 4 is drawn as if the patterns agree only on an imaginaryscreen S100 because the screen S100 that should be at a sufficientlylarge distance is drawn near the cover lens 3 due to the restriction ofthe drawing page, the actual diameter of the cover lens 3 issufficiently smaller than the proportion shown in the drawing and theluminous intensity distribution patterns substantially agree with eachother. For example, it will easily be understood that the luminousintensity distribution patterns become identical when the diameter ofthe cover lens 3 is made sufficiently smaller in FIG. 4.

As described above, light emitted from one light source creates oneluminous intensity distribution pattern (the first luminous intensitydistribution pattern) through superposition of a plurality ofsubstantially identical luminous intensity distribution patterns thatare formed by different beams emerging from the plurality of firstprisms 31.

In this specification, the first luminous intensity distribution patternformed by light that is output from one light source will be referred toas first light intensity distribution pattern of single light source, inorder to distinguish it from a luminous intensity distribution patternformed by light beams that are output from a plurality of light sources(light emitting diodes) to be described later.

(Second Prisms 32)

In this embodiment, each of the second prisms 32 has an incident surfaceconsisting of a part of a cylindrical surface. The inclination of theincident surface of each of the second prisms 32 is set such that lightthat is emitted by a light source located on the central axis of thecover lens 3 and is incident on the second prisms 32 at the center C32thereof emerges, after passing through the second prisms 32, in adirection parallel to each other and below the central axis of the coverlens 3 (schematically shown in FIG. 3 with reference numeral 102). Theradius and length of the circumference of the cylindrical surface thatconstitutes the incident surface of each of the second prisms 32 are setsuch that light that is incident on the second prisms 32 and has passedtherethrough spreads to a predetermined extent in the horizontal plane,while the vertical length of the cylindrical surface that constitutesthe incident surface is set such that light that is incident on thesecond prisms 32 and has passed therethrough spreads to a predeterminedextent in the vertical plane (plane perpendicular to the horizontalplane). The cylindrical surfaces that constitute the incident surfacesof the second prisms 32 are also formed to be substantially equal toeach other.

The extent to which light incident on the incident surfaces of thesecond prisms 32 in the horizontal direction and in the verticaldirection is set in accordance with the luminous intensity distributionpattern required of the LED lamp in order to form the luminous intensitydistribution pattern required of the LED lamp. The luminous intensitydistribution patterns of light emerging from the second prisms 32 can bemade equal to each other by forming the cylindrical surfaces thatconstitute the incident surfaces of the second prisms 32 substantiallyequal to each other and setting the inclination of the incident surfaceof each of the second prisms 32 such that light that is incident on thesecond prisms 32 at the central axis C32 thereof emerges, after passingthrough the second prisms 32, in directions parallel to each other. Inother words, the direction and shape of the incident surfaces of thesecond prisms are set so that the luminous intensity distributionpatterns of the second prisms 32 are equal to each other.

FIG. 5 shows light emerging through second prisms 32 a, 32 b of the twoprisms 30 a, 30 b that are arbitrarily selected from among the pluralityof prisms 30 (i.e., the same arbitrarily selected prisms 30 a, 30 bshown in FIG. 4). As shown in FIG. 5, in this embodiment, the luminousintensity distribution patterns formed by the second prism 32 a and theluminous intensity distribution patterns of the second prism 32 b areformed so as to have the same extent (cover the same area) in thehorizontal direction and in the vertical direction. Meanwhile, theluminous intensity distribution patterns formed by the second prism 32 aand the luminous intensity distribution patterns formed by the secondprism 32 b are substantially equal to each other in terms of thedirection and the pattern.

Although FIG. 5 is drawn as if the patterns agree only on the specificscreen S100, this is due to the restriction of drawing paper.

It will be easily understood that, actually, the luminous intensitydistribution patterns substantially agree with each other when thediameter of the cover lens 3 is made sufficiently smaller than theproportion shown in the drawing.

As described above, light emitted from one light source makes oneluminous intensity distribution pattern (the second luminous intensitydistribution pattern), that is different from the first luminousintensity distribution pattern, through superposition of a plurality ofsubstantially identical luminous intensity distribution patterns thatare formed by different beams emerging from the plurality of secondprisms 32.

In this specification, the second luminous intensity distributionpattern formed by light that is output from one light source will bereferred to as the second light intensity distribution pattern of asingle light source in order to distinguish it from a luminous intensitydistribution pattern formed by light beams that are output from aplurality of light sources (light emitting diodes) to be describedlater.

As will be understood from the above description, in this embodiment,light emitted from one light source makes a luminous intensitydistribution pattern (synthesized light intensity distribution patternof single light source) through superposition of the first lightintensity distribution pattern of a single light source and the secondlight intensity distribution pattern of a single light source, by beingoutput through the cover lens 3.

(LED Assembly 1)

The LED assembly 1 is formed by disposing the plurality of lightemitting diodes 11 on the substrate 12 in a predetermined arrangement.In this embodiment, the arrangement pattern of the light emitting diodes11 and the intensity of light emitted by the light emitting diodes areimportant elements that determine the luminous intensity distributionpattern (characteristics) of the LED lamp, along with the shape of theprism 30 and arrangement of the prisms 30.

That is, in this embodiment, the number of light emitting diodes 11disposed in the upper row is made larger than the number of the lightemitting diodes 11 disposed in the lower row, so that the intensity oflight is different between the upper and the lower portions in the lightsource of the plurality of light emitting diodes, as shown in FIG. 6.Since this enables the LED assembly to increase the intensity of lightdirected in the lower direction from the LED lamp, it is possible toeasily make the LED lamp for a traffic signal that is required to directlight of higher intensity in the lower direction by combining this withthe configuration of the prisms 30 of the cover lens 3 describedpreviously. In our research, it has been confirmed that, when the LEDlamp for a traffic signal is made by combining this LED arrangement withthe configuration of the prisms 30 of the cover lens 3 describedpreviously, it is preferable to set the intensity of light emitted bythe light emitting dioides 11 located below the optical axis of the LEDassembly 1 to 40% or less of the intensity of light emitted by the lightemitting diodes 11 located above the optical axis. Thus intensity oflight may be made different between the upper and lower portions eitherby setting the number of light emitting diodes as described above, or bychanging the current drawn by the light emitting diodes or the luminancelevel of the light emitting diodes

(Luminous Intensity Distribution Pattern Formed by Collective LightSources and Cover Lens 3)

The luminous intensity distribution pattern of the LED lamp as a wholewill be described below for a case in which the LED lamp is constitutedfrom a combination of the LED assembly 1 comprising the light emittingdiodes disposed in the arrangement pattern described previously and thecover lens 3 of the constitution described previously.

With reference to FIG. 4, the luminous intensity distribution patternsof the first prisms 31 a, 31 b shift downward, while maintaining theidentical patterns when the light source is shifted to above the opticalaxis of the cover lens 3. Also in FIG. 4, the luminous intensitydistribution patterns formed by the first prisms 31 (i.e., arbitraryprisms 31 a, 31 b ) shift to the left of the optical axis whilemaintaining the identical patterns when the light source is shifted tothe right of the optical axis of the cover lens 3. Also in FIG. 4, theluminous intensity distribution patterns formed by the first prisms 31shift to the right of the optical axis while maintaining the identicalpatterns when the light source is shifted to the left of the opticalaxis of the cover lens 3.

Even when the light source is located at a position offset from theoptical axis of the cover lens 3, the first prisms 31 have identicalfirst light intensity distribution patterns of a single light source,while the first light intensity distribution pattern of single lightsource has a direction that corresponds to the position of the lightsource relative to the optical axis of the cover lens 3. That is, whenthe LED assembly 1 comprising the plurality of light emitting diodes 11disposed in the predetermined arrangement pattern is used, the luminousintensity distribution pattern formed by the LED assembly 1 and thefirst prisms 31 (first light intensity distribution pattern ofcollective light source) is the superposition of all the first lightintensity distribution patterns of single light sources formed by theindividual light emitting diodes 11 and the first prisms 31.

As will be clear from the above description, since the first lightintensity distribution pattern of a single light source formed by anindividual light emitting diode 11 has a direction that corresponds tothe position of the light emitting diode 11, the first light intensitydistribution pattern of a collective light source formed as thesuperposition of the first light intensity distribution patterns ofsingle light sources becomes a luminous intensity distribution patternthat corresponds to the arrangement pattern of the LED assembly 1.

When the number of light emitting diodes 11 disposed above the opticalaxis of the cover lens 3 is made larger than the number of the lightemitting diodes 11 disposed below the optical axis, for example, thefirst light intensity distribution pattern of a collective light sourcehas a light intensity distribution that is stronger in the lowerportion.

Now with reference to FIG. 5, the luminous intensity distributionpatterns formed by the second prisms 32 a, 32 b shift downward whilemaintaining the identical patterns when the light source is shiftedabove the optical axis of the cover lens 3. Also in FIG. 5, the luminousintensity distribution patterns formed by the second prisms 32 (i.e.,arbitrary prisms 32 a, 32 b ) shift to the left of the optical axiswhile maintaining the identical patterns when the light source isshifted to the right of the optical axis of the cover lens 3, while theluminous intensity distribution patterns formed by the second prisms 32shift to the right of the optical axis while maintaining the identicalpatterns when the light source is shifted to the left of the opticalaxis of the cover lens 3.

As described above, similarly to the case of the first prisms 31, evenwhen the light source is located at a position offset from the opticalaxis of the cover lens 3, the second prisms 32 have identical secondlight intensity distribution patterns of single light sources, while thesecond light intensity distribution pattern of a single light source hasa direction that corresponds to the position of the light sourcerelative to the optical axis of the cover lens 3. That is, the secondlight intensity distribution pattern of a collective light source formedby the LED assembly 1 and the second prisms 32 is the superposition ofall the second light intensity distribution patterns of single lightsources and becomes a luminous intensity distribution pattern thatcorresponds to the arrangement pattern of the LED assembly 1.

When the number of light emitting diodes 11 disposed above the opticalaxis of the cover lens 3 is made larger than the number of the lightemitting diodes 11 disposed below the optical axis, for example, thesecond light intensity distribution pattern of a collective light sourcehas a light intensity distribution that is stronger in the lowerportion.

To sum up, the luminous intensity distribution pattern of the LED lampformed by combining the LED assembly 1 and the cover lens 3 in thisembodiment is the luminous intensity distribution pattern formed bysynthesizing the first light intensity distribution pattern ofcollective light source formed by the arrangement pattern of the LEDassembly 1 and the first prisms 31, and the second light intensitydistribution pattern of a collective light source formed by thearrangement pattern of the LED assembly 1 and the second prisms 32.Thus, the luminous intensity distribution pattern of the LED lamp ofthis embodiment is achieved by combining (1) the arrangement pattern ofthe plurality of light emitting diodes 11, (2) the assembly of theplurality of first prisms 31, and (3) the assembly of the plurality ofsecond prisms 32.

(Distance Between the Cover Lens 3 and the LED Assembly 1)

In the present invention, the distance between the cover lens 3 and theLED assembly 1 is not restricted by the fact that the particular coverlens 3 and the LED assembly 1 are used. However, the distance ispreferably set within a range described below in consideration of thedirectivity characteristic of the light emitting diode and the requiredsize of the LED lamp.

As shown in FIG. 7, it is preferable to set the distance between thecover lens 3 and the LED assembly 1 such that the angle between the linethat connects the light emitting diode 11 located at the outermostposition of the LED assembly 1 and the periphery of the cover lens 3 onthe inner surface thereof, and the central axis of the cover lens 3 isnot larger than 60° C. This is because an angle larger than 60° C. leadsto insufficient intensity of light incident on a part located away fromthe center of the cover lens 3 due to the directivity characteristic ofthe light emitting diode, thus making it impossible to achieve uniformplanar light emission (as the periphery of the cover lens darkens),resulting in lower efficiency of light emission.

It is also preferable to set the distance between the cover lens 3 andthe LED assembly 1 such that the angle between the line that connectsthe light emitting diode 11 located at the outermost position of the LEDassembly 1 and the periphery of the cover lens 3 on the inner surfacethereof, and the central axis of the cover lens 3 is 30° C. or larger.This is because an angle smaller than 30° C. increases the lightcollecting power but leads to a greater burden of diffusing lightimposed on the cover lens, thus resulting in lower efficiency of lightemission and a larger profile of the LED lamp.

When the LED lamp is used outdoors in such an application as a trafficsignal, there may arise such a case as sunlight is reflected in theouter surface of the cover lens resulting in decreasing contrast whenilluminated. In order to mitigate the reflection of sunlight, the outersurface of the cover lens is preferably formed as a spherical surfacehaving a radius of curvature not larger than 500 mm. Forming the outersurface with this curvature also allows it to suppress the height of cutin the prisms and improve the efficiency of light emission.

Since the LED lamp of this embodiment described above is constitutedfrom a combination of the LED assembly 1 comprising the plurality oflight emitting diodes disposed in the predetermined arrangement patternand the cover lens 3 having the plurality of prisms 30, a desiredluminous intensity distribution pattern can be achieved with a simpleconstruction. Although the predetermined luminous intensity distributionpattern is formed by using the cover lens 3 having prisms 30 thatcomprise the set of first prisms 31 and the set of second prisms 32formed on the inner surface thereof in a regular pattern and the LEDassembly 1 comprising the plurality of light emitting diodes disposed inthe predetermined arrangement in the preferred embodiment describedabove, the present invention is not limited to this constitution and maybe implemented with various modifications as follows.

Variation 1

While the predetermined luminous intensity distribution pattern isformed by using the first prisms 31, the second prisms 32, and the LEDassembly 1 that comprises the plurality of light emitting diodesdisposed in the predetermined arrangement in the embodiment describedabove, the present invention may also be embodied by using only theplurality of first prisms 31 constituted similarly to the aboveembodiment, thereby forming the predetermined luminous intensitydistribution pattern. For example, a relatively simple luminousintensity distribution pattern that is required to concentrate lightwith high intensity around the optical axis of the cover lens can beachieved only by means of the plurality of first prisms 31 as shown inFIG. 4. In this case, the arrangement of the light emitting diodes inthe LED assembly may be simple, such as concentric circles.

Variation 2

Similarly, the present invention may also be embodied by using only theplurality of second prisms 31 constituted similarly to the aboveembodiment, thereby forming the predetermined luminous intensitydistribution pattern. For example, a relatively simple luminousintensity distribution pattern that is required to concentrate lightwith high intensity in the lower portion can be achieved only by meansof the plurality of second prisms 31 as shown in FIG. 5. In this case,the arrangement of the light emitting diodes in the LED assembly may besimple, such as concentric circles.

Variation 3

Moreover, the present invention may also be embodied by using only thefirst prisms 31 and the second prisms 32 constituted similarly to theabove embodiment, thereby forming the predetermined luminous intensitydistribution pattern. In this case, the arrangement of the lightemitting diodes in the LED assembly may be simple, such as concentriccircles.

INDUSTRIAL APPLICABILITY

According to the present invention as described in detail above, it ispossible to provide an LED lamp that is capable of achieving a desiredluminous intensity distribution characteristic and producing uniformplanar light emission. Therefore, traffic signals, information displayapparatuses or the like that are capable of achieving a desired luminousintensity distribution characteristic and producing uniform planar lightemission can be made by using this led lamp.

What is claimed is:
 1. An LED lamp comprising: a plurality of lightemitting diodes; a cover lens having an inner surface with a pluralityof prisms, said light emitting diodes and said cover lens being arrangedso that each of said prisms receives light emitted by each of said lightemitting diodes, each of said prisms being shaped and arranged to emitlight received from said light emitting diodes in a substantiallyidentical luminous intensity distribution pattern.
 2. The LED lamp ofclaim 1, wherein a direction of the light emitted by each of said prismsin a vertical plane with respect to said cover lens is based on anorientation of an incident surface of each of said prisms, and adiffusion of the light emitted by each of said prisms in a horizontalplane with respect to said cover lens is based on a shape of saidincident surface of each of said prisms.
 3. The LED lamp of claim 1,wherein said cover lens has a spherical external surface with a radiusof curvature of no greater than 500 mm.
 4. The LED lamp of claim 1,wherein said light emitting diodes and said cover lens are arranged sothat an angle between a line connecting one of said light emittingdiodes located at an outermost position with respect to a central axisof said cover lens with an inner surface of a periphery of said coverlens, and the central axis of said cover lens is in a range of 30° to60°.
 5. The LED lamp of claim 1, wherein said plurality of prismscomprises a plurality of first prisms, each of said first prisms beingshaped and arranged to emit light received from said light emittingdiodes in a substantially identical first luminous intensitydistribution pattern, said inner surface further having a plurality ofsecond prisms, each of said second prisms being shaped and arranged toemit light received from said light emitting diodes in a substantiallyidentical second luminous intensity distribution pattern.
 6. The LEDlamp of claim 5, wherein said first prisms and said second prisms arearranged in pairs, each pair consisting of one of said first prisms andone of said second prisms, said pairs being arranged on said innersurface of said cover lens according to a rule.
 7. The LED lamp of claim5, wherein said first prisms and said second prisms are oriented so thatsaid second prisms emit light in a direction below a direction of lightemitted from said first prisms.
 8. The LED lamp of claim 5, wherein adirection of the light emitted by each of said second prisms in avertical plane with respect to said cover lens is based on anorientation of an incident surface of each of said second prisms, and adiffusion of the light emitted by each of said second prisms in ahorizontal plane with respect to said cover lens is based on a shape ofsaid incident surface of each of said second prisms.
 9. The LED lamp ofclaim 8, wherein a direction of the light emitted by each of said firstprisms in a vertical plane with respect to said cover lens is based onan orientation of an incident surface of each of said first prisms, anda diffusion of the light emitted by each of said first prisms in ahorizontal plane with respect to said cover lens is based on a shape ofsaid incident surface of each of said first prisms.
 10. The LED lamp ofclaim 5, wherein said light emitting diodes are arranged based on arequired overall luminous intensity distribution pattern for said LEDlamp.
 11. The LED lamp of claim 10, wherein said light emitting diodesare arranged so that no more than 40% of a total number of said lightemitting diodes are located below a horizontal plane that includes acentral axis of said cover lens.
 12. The LED lamp of claim 11, whereinsaid light emitting diodes and said cover lens are arranged so that anangle between a line connecting one of said light emitting diodeslocated at an outermost position with respect to the central axis ofsaid cover lens with an inner surface of a periphery of said cover lens,and the central axis of said cover lens is in a range of 30° to 60°.